Accurate temperature measurement has long been an objective of researchers in a variety of fields. Temperature sensing of patients is of particularly acute concern in the health care field due to the high correlation between patient health and body temperature. Indeed, a significant aggregate expenditure in support of medical care is dedicated to the accurate assessment of body temperature--in hospitals, clinics, nursing homes, doctors' offices and, of growing importance, at home. Invariably, one of the first things sought during a visit to a doctor or hospital is the patient's body temperature. As such, patient temperature assessment is a large and important expenditure in providing suitable health care.
Past patient temperature measurement systems have migrated from slow mercury thermometers (oral and rectal) to electronic contact sensors (predictive thermometers using resistive contact elements) to more recent non-contact systems based on infrared ("IR") sensing. A particularly successful clinical thermometer corresponds to U.S. Pat. No. Re: 34,789 to Fraden (the "Fraden IR Thermometer") the contents of which are incorporated by reference as if restated in full. The Fraden design utilizes a highly sensitive infrared detector engineered with a specific optic system to permit accurate assessment of radiation from the tympanic membrane of a patient's ear. The sensed radiation is converted to a temperature reading having an impressive correlation to actual patient temperature. Importantly, the above design permits accurate temperature measurement in 1-2 seconds with minimal patient discomfort.
Clinical practice mandates a sanitary environment for the patients and instruments. Disposable sanitary protective covers have become an ubiquitous commodity in patient care, minimizing the spread of infectious agents and cross-contamination of patients under care in a common area. The needs for sanitary practice also exist in the use of IR thermometers. Accurate temperature measurement with an IR thermometer requires the controlled placement of the sensor probe into the patient's ear canal for proper alignment between the IR sensor and the tympanic membrane of the ear. The ear canal is not a likely source of germs or other contaminants--however, medical care mandates the reduction of risk of cross contamination whenever possible. Additionally, the optics of an infrared thermometer must remain free and clear of ear wax. In view of the foregoing concerns, use of the minimally intrusive infrared thermometers of the Fraden type is advantageously implemented with a sanitary barrier precluding contact between the sensor and the patient's ear.
Sanitary barriers for clinical thermometers are not new. Indeed, many older clinical thermometers based on contacting mucous membranes for sensing temperature required the use of a disposable cover as a sanitary barrier, which was discarded after each use. Early sanitary cover designs were quite simple in concept. The basic structure applied a rigid wall for handling purposes combined with a thin film contiguous with the contact sensor element. The film was thin and often stretched to minimize the thermal barrier to conductive heat transfer. Exemplary early cover structures are depicted in U.S. Pat. No. 3,822,593 to Oudawaal and U.S. Pat. No. 3,987,899 to Vyprachticky. These early covers were routinely made from inexpensive plastics such as polyethylene and polypropylene and either injected molded as one unitary structure or formed in two parts--bonding a thin film onto the more rigid body portion. Either way, the resulting cover would be attached to the sensor and then the combined structure placed, e.g., in the patient's mouth to obtain the temperature reading. After the reading, the cover is detached and discarded.
Early infrared thermometers also employed disposable covers. These early disposable covers for infrared thermometers originally were closely modeled on the above-described covers for contact thermometers. For example, U.S. Pat. Nos. 5,293,862 and 5,179,936 to O'Hara, et al. disclose a two-piece cover design wherein a thin transparent film is bonded to a rigid tubular body forming a disposable cover for an infrared thermometer. The manufacturing process of this cover design causes the formation of wrinkles in the thin transparent film. Film wrinkles of this nature may interfere with infrared transmission between the tympanic membrane of the ear and the IR sensor of the thermometer. Accordingly, the use of this probe cover design required the removal of these wrinkles by stretching the film over the infrared sensor.
However, stretching the film to remove wrinkles may create other sources of potential measurement inaccuracies. Firstly, a film that is stretched may stretch in a non-uniform manner creating a "lensing" effect that may distort transmitted infrared radiation. Secondly, the stretched film may result in a realignment of the polymer molecular structure causing variations in both the reflective and absorptive properties of the film. Accordingly, such prior art infrared probe covers that require stretching to remove wrinkles or other undesirable surface characteristics in the film window of the disposable cover may possess somewhat unpredictable transmission properties when fitted onto a probe of an infrared thermometer.
Similarly, U.S. Pat. No. 4,911,559 issued to Meist and Suszinski discloses an infrared probe cover susceptible to the effects of stretching of the film. The Meist and Suszinski patent teaches a laminated probe cover where the polymer film significantly stretches when being fitted over an infrared thermometer probe. The effects caused by this stretching of the film invariably affects the transmissivity of the film in unpredictable ways that may result in errors in temperature measurement.
Other problems have arisen with past designs of probe covers for infrared sensors. For example, prior art infrared probe covers were often configured so that the film would contact the patient's skin. The patient's skin--if at a different temperature than that of the probe cover--will cause a temperature gradient to form in the film as heat conduction triggers an energy flow from the warmer ear to the cooler film. The resulting elevated temperature of the film amplifies an error source known as secondary radiation. Secondary radiation refers to the infrared radiation that the cover emits relative to the infrared radiation emitted by the primary source, viz., the tympanic membrane of the ear. It has been found that it is important to minimize the fluctuations of secondary radiation from all sources including that of the cover film window. Secondary radiation triggered by unpredictable heat flows from the ear canal to the film window may induce measurement inaccuracies in the clinical thermometer.
An additional source of measurement error resides in the departure from good optical alignment that characterized earlier cover designs. For example, the prior art covers with non-uniform film windows (caused by either the manufacturing process or subsequent stretching) are haphazardly placed on the sensor with no ability to control the alignment of the film window relative to the sensor--target optical axis. Consequently, if the probe cover is not properly aligned or centered, the transmission of infrared radiation may be affected due to variations in the film.
Experience in clinical thermometer use has shown that it may be desirable to increase the angle of view of an infrared sensor. It is believed that the enhanced angle of view compensates for directional errors that may arise if the clinical thermometer is not properly inserted in the ear canal for a measurement, i.e., the sensor is not properly aligned with the tympanic membrane as incorrectly aimed. Probe cover designs that excessively limit the field of view of the IR sensor may become an impediment to proper temperature assessment.
As can be seen from the above discussion, the design of infrared transmissive disposable covers is a complex and difficult task. The recognition of the need for a fully functional sensor cover design and the inherent problems of the prior art have led to the present invention.